Fluid flow control apparatus



April 18, 1939- K. D. McMAHAN 2,155,279

FLUID FLOW CONTROL APPARATUS Filed May 22, 1935 2 Sheets-Sheet 1 Pas iiive Pressure Pressure a Fl 4-, g 8.0 fg 7.0 1' 5 5 6-0 lnducea draft ,6-g 5 0 COHd/t/OI? .6 Pos/t/i/e Pressure 4.0

4 Operation (Han on) Tota/ Pressure Drop Across 30 AIR FL 0w rfifiausllAF/AIMTUS QEM.

Nega't/ve fies-sure Operation (flan off] I t Q n \len OT Kenton D. McMaham ttorn ey Static fiessure-lncfies 0f [Water April 1939- K.DxMcMAHAN 2,155,279

FLUID FLOW CONTROL APPARATUS Filed May 22, 1935 2 Sheets-Sheet 2 Fig. 5.

Inventor: Kehbovw D. Mc Mahann Hi s Attorngy Patented Apr. 18, 1939 vPATENT OFFICE 2,155,z79 FLUID mow ooN'moL APPARATUS Kenton D. vMcMahan,

signer to General Electric ration of New York Application May 22,

5 Claims.

This invention relates to fluid flow control apparatus and particularlyto improved variable impedance fluid flow control apparatus forcontrolling automatically a flow of a fluid in response to variation inthe conditions producing the flow. I The result accomplished by theapparatus of the present invention is in effect the exact converse ofthe result accomplished by the invention set forth in my previousapplication, Serial No. 7e17,091, flied March 23, 1934, whichapplication is assigned to the assignee of the present invention.

In order that the present invention may be clearly understood, thefollowing principles are set forth: Bernoulli's theorem for fluid flowstates that for any point in a fluid flow system Static head+velocityhead+summation of losses=constant the magnitude of the head lossesexisting under different conditions. It is well known in the art that.losses may be introduced into a fluid system in two ways: First, by theintroduction of a sharp-edged or similar form of orifice having a loworifice coeflicient; and second, by the introduction of a too suddenincrease in the area of the flow path. In the first case. theloss is dueto the contraction of the fluid stream on the discharge side of theorifice. In the second case, the loss depends upon the efficiency ofconverting velocity head into static head.

In the practice of my present invention, I provide in a fluid system aduct and a plurality of orifices in such cooperating relation therewiththat under one set of pressure conditions fluid I is delivered from asingle source, or one of two independent sources, through certain of theoriflces to the duct in such a manner that a minimum of contraction ofthe fluid stream is produced and the efliciency of conversion ofvelocity head to static head is a maximum; and that, under another setof pressure conditions, fluid is delivered from the single source, orfrom the other of the two independent sources as the case may be,through certain others of the orifices to the duct in such manner that amaximum of contraction of the fluid stream is produced and theefficiency of conversion of velocity head to static head is a minimum.

Of theyarious embodiments of which my invention is capable, the onesherein illustrated and specifically described have been selected asexhibiting the invention in its most convenient and effective form, andas forming the basis for in- I suring a "complete, understanding of theprinciples-underlying the invention in order that one Schenectady, N.Y., as- Company, a corpo- 1935, Serial No. 22,751. (01. 110-62) andproperly adapt the illustrated embodiments to various conditions ofoperation.

With particular reference to one of the illustrated embodiments, aspecific object of my invention is the provision of furnace draftcontrol apparatus wherein substantially no impedance is imposed to thesupplying of air to the furnace under forced draft and a relatively highdegree of impedance is imposed to the supplying of air to the furnaceunder'induced draft.

With reference to the other of the illustrated embodiments, a specificobject of my invention is the provision in a ventilation system for agiven I enclosure the temperature of which it is desired to control, ofimproved apparatus for automatically controlling the proportions offresh and recirculated air supplied to the enclosure in accordance withvariations in the temperature within the enclosure.

I I f Further objects of my invention and the details This equationindicates that flow through a" fluid system may be regulated bycontrolling of the manner in which the invention is carried out will bemade clear. by the following description taken in conjunction with theaccompanying drawings in which:

Figs. 1 and 2 diagrammatically illustrate in longitudinal section thegeneral form and arrangement of apparatus for practicing my invention,the form of fluid flow under different conditions being represented inthe figures by arrows; 4

Figs. .3 and 4 represent curves of certain operational characteristicsof apparatus such as that illustrated in Figs. 1 and 2;

Fig, 5 diagrammatically illustrates an embodiment of my invention asapplied to furnace draft control, and

Fig. 6 diagrammatically illustrates another embodiment of my inventionasapplied to the control of the recirculation of fiuid in a coolingsystem.

Referriugito Figs. 1 and 2, iii designates a fluid receiving casing inthe interior C of which a negative pressure (pressure below atmospheric)may be produced by any suitable form of suction device (not shown). i irepresents a duct for supplying-Zfluid, such as air, to the interior ofthe casing'dii. This duct may assume various forms in the practice of myinvention but is illustrated in ,its preferred embodiment as being inthe form of a truncated cone having smooth walls diverging from itsinlet or throat it to its outlet or mouth i2, which is in communicationwith the interior of the casing in. The duct i l constitutes one form ofa diffuser which is a device well known in the art for convertingvelocity head of a fluid stream into static head. In order that such adiffuser be ei'licient in its operation, its structural characteristicsmust be such that the outer portions of a fluid stream flowingtherethrough follow along the sides'thereof in the form of substantiallycontinuous unidirectional streams. In other words, the circumferentialflow must be substantially laminar without the formation of eddies alongthe inner surfaces of the diffuser. To make the fluid follow the sides,the area must increase very gradually, and there be no conditionspresent which cause eddies to form at the inlet of the diffuser. Inaccordance irith well known principles, the walls of the dif- Idserillustrated are made to have an angle with the axis of the diffuserwhich is less than that at which the outer laminae of a fluid treamentering the throat 13 substantially uniformly distributed across thearea of the throat with all portions thereof flowing at substantiallyequal velocity and along lines substantially parallel to the axis of thediffuser, will cease to contact the walls of the diffuser.

Ehe throat l3 of the diffuser II is provided with an inlet orifice bwhich has a sharp edge on the inlet side thereof and is therefore of thetype commonly designated as sharp-edged. It well known in the art thatsuch sharp-edged "rifice has a relatively low orifice coeflicientfor thenormal flow of fluid therethrough from a body of fluid which is inextended contact with the margin of the orifice on the inlet sidethereof. Eddy currents are produced at the inlet of the diffuser by therapid increase in the area of the actual fluid flow path through thediffuser which is introduced by the contraction of the fluid streamentering the throat l3 at an angle to the axis of the diffuser throughthe, sharp edges of orifice b.

Numeral l5 designates a supply casing I! the interior space A of whichmay be subjected to positive pressure (pressure above atmospheric) byany suitable means such as a blower .(not shown). The casing I5 isprovided with a discharge opening a which is separated from the throatof the diffuser II by a space designated on the drawings as S, and whichis in axial alignment with the diffuser in order that fluid may beprojected from the casing through the opening a into the orifice b. Forreasons hereinafter to be given more fully, it is preferable that theopening a be of the round-edged type, which has a relatively highorifice coefllcient, and also that the orifice a have a slightly largerarea than that of the orifice b forming the inlet of the diffuser andhave a circular cross-sectional shape to correspond to the circularcross-section of the diffuser.

For the purpose of describing the operation 0 the apparatus illustratedin Figs. 1 and 2, it will be assumed that the'apparatus is to beemployed to control the supply of air to the space C interiorly of thecasing I0 from the space A interiorly of the casing l5 and from theatmosphere, designated as the space B surrounding the apparatus. If theinterior of casing I5 is subjected to positive pressure, air isdischarged therefrom through the orifice a into the throat 13 of thediffuser ll. Also, if the interior of casing I0 is subjected to negativepressure, there is a tendency for air to be drawn into the throat of thediffuser through the space S from atmosphere, and, as will be pointedout hereinafter, the amount of air drawn in from atmosphere dependsprincipally upon the difference between the pressure of space A and ofspace B.

The operation of the apparatus will be more clearly understood byreference to the arrows shown in Figs. 1 and 2. When the interior C ofcasing in is under negative pressure and the interior A of easing I5 isunder positive pressure, a stream of air is discharged through orifice ainto the throat I3 of the diffuser II as shown by the arrows in Fig. lwhich are representative of the'fiow lines of the air stream. Since inthe illustrated embodimentthe orifice same cross-sectional shape,-therim of the orifice b operates to core out the central portion of thestream discharged from the orifice a. and the outside portions orlaminae of the stream are discharged outwardly through the space S andflow along the outer wall of the diffuser. This outer portion of the airstream flowing on the outside of the diffuser overcomes or blocks anytendency for air to be entrained into the inlet of the diffuser from theatmosphere B through the space S by means of the injector action of thestream flowing through the inlet of the diffuser. Consequently, thecentral portion of the stream flows into the inlet of the diffuser andis substantially uniformly distributed across the area thereof with allportions of the stream flowing at substantially uniform velocity andalong lines substantially parallel to the axis of the diffuser. Byproper adjustment of the sizes of orifices a. and b and of space S withrelation to each other, the discharge of air externally of the throat ofthe diifuser may be reduced to a minimum while the blocking efl'ect a isslightly larger than the orifice b and has the thereof is maintained.Under these conditions the orifice a in effect becomes the inlet of thediifuser since practically all of the air discharged from the orifice aenters the inlet b of the diffuser.

Under the conditions of flow at uniform velocity and along linessubstantially parallel to the axis of the inlet b outlined in theprevious paragraph the inlet orifice coemcient of the diffuser is only alittle less than unity. This is due to the fact that, the stream of airenters the throat of the diffuser with very slight loss in head and withsubstantially no contraction. Also the diffuser being designed aspreviously pointed out so that it has maximum efiiciency of operationwhen a fluid stream enters the throat thereof as just described, therebeing no eddy currents introduced at the inlet thereof and the fluidstream flowing smoothly therethrough as indicated by the arrows in Fig.l, the loss in head across the diffuser is a minimum. Consequently,under the positive pressure conditions a minimum of impedance is imposedto flow of air from the interior of the casing IE to the interior of thecasing l0, and for a given pressure drop across the apparatus a maximumfluid fiowis secured.

On the other hand, 'when the interior A of the casing I5 is underatmospheric pressure and the interior 0 of the casing I0 is undernegative pressure, a stream of air is no longer discharged from theorifice a into the throat I! of the difl'user II in the mannerpreviously described and the flow of air into the throat of the diffuserfrom atmosphere B through the space S is no longer blocked off. Theorifice b becomes the effective inlet of the diffuser and the air flowsfrom atmosphere into the diffuser at a relatively large angle to theaxis thereof, as indicated by the arrows in Fig. 2,-

so thatthe low orifice coefilcient at the diffuser inlet results in anappreciably larger loss in head at the inlet than was the case under thepositive pressure conditions outlined in the previous paragraph. Inaddition the contraction of the air stream entering the diffuser throughthe orifice b produces eddy currents along the walls of the difl'user,as indicated by the arrows, which impede the flow of air through thediffuser and cause inefficient operation of the diffuser and aconsequent large loss in head thereacross. The total loss in head acrossthe apparatus under the negative pressure conditions due to the effectof the I sharp-edged orifice and the inefllcient operation of thediffuser-is many times larger than the total loss across the apparatusunder the positive pressure conditions. j

In Figs. 3 and 4 are illustrated curves which represent the results oftests and calculations based on operation of flow control apparatusconstructed in accordance with my invention. From these curves it may beascertained that the magnitudes of negative pressure required to producegiven rates of air flow through the apparatusare I in the neighborhoodof 18 to 20 times greater than the magnitudes of positive pressurenecessary to produce the same rates of air flow through the apparatus.-

When the space C is subjected to a constant negative pressure at thesame time that the space A is subjected to a variable positive pressure,as-

the pressure within the space Adecreases the force of thedischargethrough the orifice. a also decreases with'resultant decrease in theblocking effect produced thereby at the space designated S. A point isreached at which air begins to flow 1 portion to a, function of thediflerential.

From an understanding of. the principles underlying my invention, asset-forth in the fore going discussion, it will be evident to oneskilled in the art that the invention is not limited to the illustrateddetails of the discharge orifice a, space S, and supply duct II, andthat the apparatus for practicing the invention may take various formswhereby may besecured diflerent degrees of variation in impedance orrestriction of the fluid flow through the apparatus upon changeover frompositive to negative pressure conditions or vice versa.. It will also beevident that the invention may be employed with equal advantage incontrolling the flow of a fluid other than airfin which case the space Awithin the casing i! may be supplied with fluid of one kind and thespace 13 may constitute a second source of fluid of a different kind incommunication with the inlet of the duct II by any suitable meanscorresponding to the space 8. The terms "positive pressure" and"negative pressure" are herein usedin a relative sense since, whenreferred to the scale of absolute pressure, pressures above and belowatmospheric are in effect positive pressures of diflerent magnitudes.Keeping this in mind, it is evident that apparatus constructed inaccordance with the present invention will function equally well incases where the space C is subjected to a given positive pressure whilesp'aces A and Bare subjected to given higher positive pressure and thepositive pressure of space A is varied relative to that of space B.

In Fig. 5 an embodiment of my invention is iliustrated as employed forcontrolling the draft or supply of combustion air to a furnace itprovided with a flue or stack connection I! which in accordance withwell known principles is eflective during operation of the furnace toproduce an induced draft through the furnace. a blower which is employedfor supplying forced I8 designates draft to the furnace. The blower isdriven in any suitable manner as by means of electric motor ii. In theapplication of the invention to the furnace draft control. it is desiredto produce (1).

and (2), veryhigh impedance to flow of air through the furnace undernegative pressure produced by the stack. It is desired to have minimumhead losses incident to'the supply of combustion air to the furnaceunder forced draft in order that the blowerunit producing the forceddraft may be as small as possible. However, if

the air flow'into the furnace when produced by induced draft only,follows the same course as that produced by operation of the blower, theloss in head is of course still small. This is considered objectionablebecause the air flow caused by natural draft when the stack is stillhot, immediately after shutdown of the blower and the furnace,

' low impedance to flow of air through the furnace under positivepressure produced by the blower.

would approach that caused by forced draft.

This would result in too rapid burning of fuel within the interior ofthe furnace after shutdown thereof and would lead to overshooting. Itis, therefore, desirable to have a minimum of impedance 'or restrictionimposed to the supplying of air to the furnace by means of the blowerand a maximum of impedance or restriction imposed to the drawingv of'airthrough the furnace by means of the induced draft producing stack whenthe blower is not in operation.

Referring again to Fig. 5, ii designates the V supply duct'or diffuserof my invention mounted in any suitable manner, as by means of boltsindicated at 20, with its mouth or outlet in sealed communication withthe draft opening of the furnace it. The nozzle of the blower i8corresponds to the previously referred to casing i6 and is sodesignated. This nozzle is provided with round-edged discharge orifice ain spaced axially aligned relationship with the inlet oriflce b of thediffuser H.

The flow control apparatus illustrated in Fig. 5 operates in the samemanner as that illustrated in Figs. 1 and 2. When the furnace isinoperation and the blower I8 is operating to supply combustionairthereto, a stream of air is discharged under pressure from the nozzlecasing i5 and orifice a into the .inlet orifice b of the difluser ii andthence through the diffuser into the interior-of the furnace. Aspreviously outlined, under these conditions the impedance imposed toflow of air through the supply duct or difluser is a minimum and hence amaximum amount of air is forced therethrough by a given pressure dropcreated by operation of the blower. Upon shutdown of the fiu'nace andcessation of operation of the blower, the space 8 is no longer blockedoff by the discharge from the orifice a and air flows therethrough fromthe atmosphere v into, the inlet oriflce b of the diffuser, therebyproducing an impedance to the flow of air through the diffuser which ismaterially greater than that which would be imposed by the inoperativefan. Under these last conditions the rate of flow of air through thestack is a minimum.

for a pressure drop corresponding to the given action of the draftinducing stack II. It will and, since thus be seen that the flow controlapparatus functions automatically and without the use of moving parts asa damper for producing very sensitive regulation of the supply of alr'tothe furnace.

As an aid to a-complete understanding of the method of constructing animproved flow control apparatus in accordance with my invention, thefollowing calculations are given as illustrative of the manner ofderiving the mathematical equations upon which such constructiondepends. It will be assumed that it is desired to know thecharacteristics of apparatus such as that illustrated in Figs. 1, 2 and5, and, further, it will be assumed that:

l. The change in density of the air during its fiow through theapparatus is negligible. This assumption is ordinarily used in dealingwith problems of air flow in which pressure changes are small andvelocities involved are comparatively low, such as in machineventilation problems.

2. Atmospheric conditions are assumed as,

Temperature=70 F. Pressure =14.7 lbs. per sq. in.

3- Gravity effects are negligible. The following nomenclature will beused in the calculations A2=Orifice area in sq. ft. This denotes theorifice a which is equivalent under particular conditions to the inletof the diffuser. As previously stated, orifice a, is the equivalentinlet under positive pressure conditions and orifice b is the effectiveinlet under negative pressure conditions. I

A2=Area at outlet end of diffuser in sq. ft.

Q=Discharge in cubic ft. per minute.

=Density of air.

C=Oriflce coefficient.

E=Diifuser efilciency which may be defined as the ratio of the change ofstatic head 'to the change of velocity head through the I diffuser;

h=Velocity head in inches of water.

p=Static pressure head in inches of water.

H=total head=p+h.

subscripts l, 2, and 3 denote conditions before entering orifice A2.immediately after leaving orifice A2, and at the outlet of the diffuserrespectively.

Under the atmospheric conditions assumed the orifice coefficient C maybe determined from the following equation:

where 4 Q=Air flow in cubic feet per minute,

A=Area of orifice in sq. ft.,

P=Pressure drop across the orifice in inches of water. This equation isderived from the standard fluid flow equation V= /2gH and the constant4010 represents the combination of all the conversion factors involved.

Also for the atmospheric conditions assumed, the velocityhead may bewritten:

a j (l) h=( inches of water,

a (2) h=( inches of water.

Directly from the definition of diffuser emciency the total loss of headin the diffuser may be written:

inches of water.

In the equation above given for determining the orifice coefficient andwhich is as follows:

the quantity P" is the total loss in head across the orifice plus thedynamic head on the exit side,

' I (5) P= H inches of water.

Directly from (4) and (5) the total loss in head across the orifice is I(6) l2=( (-l) inches of water.

Adding Equations (3) and (6), thetotal loss of head is inches of water.

The calculations of pressure drop for various values of Q may be madedirectly from Equation (7). It will be seen that the only difference inthe Equation (7) for the cases of the blower operating and not operatingis in the values of 0" and "E", since for the purposes of thesecalculal2), 4 in.; angle of walls of diffuser with axis thereof, 7. Withreference to this last dimension, it has been found that the angle of 7approaches the maximum value for which the diffuser operates efficientlywithout eddy currents being formed when fiuid is discharged into theinlet thereof in the form of a stream substantially uniformlydistributed over the inlet with all portions of the stream flowing atsubstantially uniform velocity and along lines substantially parallel tothe axis. From data familiar in the art the values of the orificecoefficients may be found to approximate 0.98 for the round-edgedorifice a and 0.60 for the sharp-edged orifice b. A spacing of V in.between the two orifices is provided. From empirical data it may bedetermined that the efilciency of a diffuser having the abovecharacteristics is approximately 90% when a stream of air is dischargedinto the inlet orifice b in the manner above described, and isapproximately 25% when air is being drawn into the inlet orifice I)through the space S.

Then, taking the numerical values of the diameters of A: and A: as 2 in.and 4 in. respectively, Equation (7) may be reduced to:

inches of water.

As previously stated, when the blower is operating the round-edgedorifice a is the eflective inlet of the diffuser and this has an orificecoeflicient C=0.98. Also under these conditions the value ofthe diffuserefliciency E=0.90. Using these numerical values, the equation for thetotal head loss across the apparatus with the blower operating is asfollows:

(9) 1"3=1.77 10 Q inches of water.

When the blower is not operating, the orifice b becomes the effectiveinlet of the diffuser and the orifice coefficient C thereof, aspreviously stated is 0.60. Also from the data previously given thediffuser efficiency E under these conditions equals 0.25. Using thesenumerical values in Equation (8). the equation for the total lossesacross the apparatus is as follows:

(10) 1' 3=3.24 10- Q inches of water.

From Equations (9) and (10), curves such as those represented in Fig. 4may be plotted to determine the values of pressure drop corresponding torates of air fiow ranging between zero and 150 cu. ft. per minute, bothwith the blower operating and with the blower not operating in the caseof furnace draft control employing apparatus having the dimensions abovegiven.

From Equation.(8) using the dimensions specified, it will be seen thatthe total loss in head across the apparatus under each of the operatingconditions is proportional to the quantity Substituting the values of Cand E whiclr vsrlere specified for the conditions incident to operationand non-operation of the blower, it is found that with the blower notoperating the pressure drop across the apparatus is 1 83 times thepressure drop across the apparatus. with'the blower operating for thesame rates of *air fiow. Or, in other words, for a given pressure dropthe rate of air flow with the blower operating is 4.27 times the rate offlow with the blower not operating. Thus it may be seen that when theblower is stopped the air flow decreases to less than 24% of its formervalue.

A second-embodiment of my invention is diagrammatically illustrated inFig. 6 as employed for controlling the recirculation of air in a coolingsystem. The element designated by the numeral 2| represents an electricdischarge device which is excited from electrical supply lines 22 andwhich gives on heat during operation. It is to be understood that such adevice is shown for purposes of illustration only and there may besubstituted therefor any other heat dissipating device the temperatureof which it is desired to maintain within given limits by thecirculation of air thereover. The heat dissipating device is enclosed bymeans of casing 23 which is provided with intake and exhaust meansillustrated as being in the form of ducts 24 and 25 respectively. Theseducts are so constructed. that their open leads are in spaced apartopposed relation. The

duct 24 is provided with an inwardly converging bell-shaped inletopening 26 which facilitates the smooth flow of air thereinto. Hdesignates a supply duct or diffuser having the characteristicspreviously described. This duct is mounted with its mouth or outlet l2in opposed relation to the inlet 26 of the duct 24 and is spacedtherefrom as indicated at 21. For best operation of the apparatus it ispreferable though not necessary that the outlet 1 2 of the diffuser beof equal or slightly greater area than that of the inlet 26 of the duct24. The throat l3 of the diffuser extends into atmosphere and, aspreviously described, is provided with sharp-edged inlet orifice b.

In opposed relation to the outlet 30 of the exhaust duct 25 and spacedtherefrom as indicated at 3i is the inwardly converging bellshaped inlet32 of a fluid conduit 33 the outlet of which is in the form of around-edged discharge orifice a. 'in axial alignment with the diffuser Hand spaced from the inlet thereof as indicated at S. It is preferablethough not necessary that the inlet 32 of the conduit 33 have a slightlygreater area than the outlet 30 of the exhaust duct 25.

In order to circulate air through the casing 23 and over the heatdissipating device 2|, a fan 34 is provided in the intake duct 24adjacent the inlet 26 thereof and is driven by means of electric motor35, or other suitable means, which is connected to be energized fromelectric supply lines 36. A similar fan 31 is located in the conduit 33adjacent the inlet 32 thereof and is arranged to be driven by means ofelectric motor 36, or other suitable means, which is also connected tobe energized from the supply lines 36. Motor 35 is connected directly tothe supply lines 36 in order that when energized it rotates at constantspeed to force a constant volume of fluid into the intake duct 24 andthrough the casing 23. On the other hand, it is desirable for reasons tobe given hereinafter that the fan 31 be operated at different speeds andhence the motor 38 is connected to the supply lines 33 through avariable resistance or similar speedcontrol device 40, or similardevice, and movable contact 4|. The contact 4i is mounted on the movableand of thermostatic device 42 which is represented as being of theexpansible fluid bellows type having one end 43 thereof extending withinthe casing 23 in proximity to the electrical discharge device 2| to beresponsive to the heat dissipated therefrom during operation. The

resistance 40, contact 4| and thermostatic device 42 constitute athermal. responsive control device for the motor 38. Both the motor 35and the motor 33 may be disconnected from the supply line 36 by means ofthe manually operable switch 44.

The purpose of the apparatus illustrated in Fig. 6 is to maintain theelectric discharge device 2| at a certain constant temperature or withina certain temperature range regardless of the ambient temperatureoutside the casing 23. This is accomplished by varying the. proportionsof recirculated air and make-up air supplied to the casing by the fan34. Under certain conditions the amount of recirculated air must besubstantially zero and for other conditions the amount of make-up airmust be substantially zero. Intermediate the extreme conditions it isdesirable to obtain variations of the relative proportions ofrecirculated and make-up air in accordance with variations inthe loadconditions on the electric discharge device and variations in theambient temperature surrounding the casing 23. Fan 34 and motor 35operate at constant speed, when the switch 44 is closed, to supply aconstant volume of cooling air through the system. The function of thefan 31 and motor 38 is to control the proportions of recirculated airand fresh air supplied to the casing 23. The energization of the motor38 and the speed of operation of the fan 31 are controlled by means ofthe cooperative action of the variable resistance 40 and movable contact4| which is movable in accordance with temperature variations within thecasing 23.

The thermostatic device 42 is so adjusted that when the electricdischarge device 2| is operating at a proper temperature, all of theresistance 40 is cut out and the motor 38 receives maximum excitationwhich results in the rotation of the fan 31 at maximum speed. Underthese conditions the fan 3! draws a large volume of spent orrecirculated air from the exhaust duct 25 and discharges this airthrough theorifice a into the inlet orifice b of the diffuser l l insuch manner, as previously described, that the impedance imposed to thefiow of the recirculated air through the orifice a and diffuser I l is aminimum. The recirculated air is therefore supplied in large volumes tothe outlet l2 of the diffuser and constitutes the major por tion of theair drawn into the intake duct 24 by means of the fan 34 so that a verysmall volume of makeup or fresh air is drawn into the inlet 26 of theduct 24 from atmosphere through the space 21.

On the other hand, when the temperature of the electric discharge device2| reaches a predetermined high limiting degree, the thermostaticelement 42 operates to insert a maximum of the resistance 40 or tocompletely open the circuit of the motor 38 so that it will operate atits lowest speed or not at all. Under these conditions the fan 31 andrestricted orifice a offer such impedance to flow of air through conduit33 that substantially all of the spent or recirculated air from thecasing 23 is discharged from the outlet 30 of the duct 25 through thespace 3| into atmosphere. Since the fan 31 no longer operatestodischarge a stream of air through the orifice a into the orifice b,the space S is no longer blocked oil and air flows therethrough into theinlet orifice b of the diffuser H at an angle to the axis of thediffuser and, as previously described, imposes a high impedance to fiowof air through the diffuser and thus decreases the volume of air passingfrom the duct 25 through the conduit 33 and the diffuser l l to theinlet 26 of the duct 24. Hence, since the impedance to flow of airthrough the diffuser is now at its maximum, the fan 34, in order toforce a constant volume of cooling air through the duct 24 and casing23, draws a large volume of make-up air from atmosphere into the ductthrough space 21, thus providing maximum cooling of the electricdischarge device 2 I.

For temperatures of the electric discharge device intermediate thelimits previously mentioned, the thermostatic element 42 operates toinsert varying amounts of the resistance 40 into the circuit of themotor 38 which results in the operation of the motor at variable speedsand the fan 31 is effective to discharge varying amounts of spent orrecirculated air through the orifice a into the inlet of the diffuser ll. Under these conditions the impedance to flow of spent air through thediffuser varies substantially inversely in proportion to the amount ofsuch air which is discharged from the orifice a, and the amount ofmake-up air drawn into the inlet 2'8 of the duct 24 through space 2!varies substantially inversely in proportion to the amount of airpassing through the diffuser. Hence it will be seen that the flowcontrol apparatus of my invention functions to provide a very sensitivecontrol of the relative proportions of recirculated and make-up airsupplied to the casing 23 for cooling the electric discharge device 2|,and accomplishes this control without employing the usual movable vanesand shutters.

While I have shown and described my invention in connection with certainspeciflc embodiments, it will, of course, be understood that I do notwish to be limited thereto, since it is apparent that the principlesherein disclosed are susceptible of numerous other applications, andmodifications may be made in the arrangement and structure of theelements of the fluid flow control apparatus without departing from thespirit and scope of my invention as set forth in the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. A variable impedance fluid flow control apparatus including incombination a fluid receiving casing, a frustro-conical diffuser ducthaving its outlet in communication with the interior of said casing andits inlet in the form of a sharp-edged orifice, negative pressureproducing means operable for drawing fluid into said casing through saidduct, a conduit having a circular round-edged orifice in spaced apartaxial alignment with the inlet orifice of said duct, said last orificehaving larger area than said inlet orifice, and positive pressureproducing means connected with said conduit for discharging a stream offluid from said second orifice through said inlet orifice into saidduct.

2. The combination with a furnace having a draft intake opening andmeans for inducing a draft of air through said opening into the interiorof the furnace, of a frusto-conical difl'user duct having its outlet insealed communication with said opening and having an inlet in the formof a sharp-edged orifice for imposing relatively high flow impedanceonly to the suction of air through said diffuser duct, means providing asubstantially circular round-edged orifice in spaced axial alignmentwith the inlet of said duct, said circular orifice having agreater areathan that of said inlet, and means operable for discharging a stream ofair from said circular orifice into the inlet of said duct withrelatively low flow impedance to the discharge.

3. In combination, fluid suction apparatus having a tapering difiuserfluid inlet passage for imposing relatively high flow impedance only tothe suction of fluid into said apparatus through said passage and fluiddischarge means having an orifice in overlapping spaced apart alinementwith the inlet of said diffuser for discharging fiuid under pressureinto said apparatus with relatively low flow impedance to the discharge.

4. In combination, apparatus having a fluid fiow control tube with asharp edge inlet orifice, fluid discharge means having a dischargeorifice in overlapping spaced apart alinement with said inlet orificefor discharging fluid under pressure thereto in the form of a streamflowing through said inlet oriflce in lines substantially parallel tothe axis of the tube, and fluid suction means for drawing fluid intosaid tube through the space between said inlet orifice and saiddischarge orifice in the form or a stream having a relatively largecontraction as it flows through said inlet orifice.

5. In combination, an elongated fluid flow control tube having a sharpedge inlet orifice, fluid discharge means having a discharge orifloe inspaced apart overlapping relation withsaid inlet orifice for supplyingfluid thereto with relatively low impedance to the flow through saidtube only 5 upon operation of said fluid discharge means, and suctionmeans connected with the outlet of said tube for drawing fluid throughthe opening between said oriflces into said tube with relatively highimpedance to the flow of fluid through the tube only upon independentoperation oi! said suction means.-

, KEHTON D. McMAHAN.

